As I constructed the cabinet entirely based on the speakers properties, now should I leave the inside of the cabinet free or...may I stuff there a small carpet in "U" shape, going around the vertical part of the vent?Is it necessary? Or do I just leave like it is?

Most (well designed) speakers have something to dampen internal reflections. Some designs work best stuffed, and some work best with some or all of the internal panels lined. Carpet is probably not the best material, but if it's what you have, give it a try. Fibreglas insulation is often used, and there are other materials specifically made for speakers (maybe acoustically/technically better and probably less "nasty" to work with).

At this point it's probably best to experiment. Line the internal walls of one speaker and leave the other as-is, and A/B them. Then maybe stuff the "rejected" speaker and compare it to the speaker that "won" the previous A/B test.

Are these to be accoustic suspension or is there a bass reflex port? If sealed, the use of fibreglass effectively increases internal volume (I know, sounds backwards but go look it up), which increases bass response. Your decision should be based on where they will be used and whether that location enhances bass or not. (live room vs dead room, in the corners vs well out in the room, etc etc)

I like the look of that cabinet finish! Just curious, what driver do you plan to use?

Oh the cabinet I drew it on LibreCAD and a wood plyer worked it out for me...It's MDF 9 milimiter, polished and with 2 layers of marine veneer sprayed. Now I notice it may look bigger in the picture, but it is just 22cm tall by 14cm wide.

I have another pair that I will be doing some "serious" work with crossover, and tweeter.

This is an experimental start up. It will use coaxial speakers, so there will be no driver, just a capacitor to optimize it to 40W RMS.

Haha sorry I missed the vented part, going the bass reflex route certainly complicates things, as your port tuning has to take any internal damping material into account...which is why I'm not a huge fan of them...but in this case I would still say to use some internal damping, it'll keep them from having a 'hollow' sound, but somehow find a way to experiment with your port tuning, use a paper towel roll or some pvc pipe you can add pieces to in order to 'tune' the port to get the response you want, as once you've added damping to the internals it does change the formulas for port length/diameter.

This is an experimental start up. It will use coaxial speakers, so there will be no driver...

FYI - the term "driver" refers to a speaker without a cabinet. You'll be using coaxial drivers. Sometimes drivers are called speaker components. So you could have a woofer driver and a tweeter driver, or a woofer component and a tweeter component, etc.

The term "speaker" is ambiguous and can mean either a speaker without a cabinet, or a speaker system which means one or more speakers/drivers in a cabinet. If you go to an audio/video store and buy a speaker, you are probably buying a speaker system in a cabinet. But if you go to an electronics-parts store, or a car stereo shop, and buy a speaker, you are probably buying just the driver.

When a guitar player says "cabinet" (or "cab"), he's talking about a cabinet with speakers (drivers) that requires a separate "head amp".

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It will use coaxial speakers, so there will be no driver, just a capacitor to optimize it to 40W RMS.

Most coaxial speakers (drivers) come with a capacitor already attached and wired in series with the tweeter. I've never seen a case where you had to supply your own or wire it up yourself. Usually, you just attach the two speaker wires and you're done. (High-end coax drivers sometimes have a full-crossover attached or supplied.)

As I constructed the cabinet entirely based on the speakers properties, now should I leave the inside of the cabinet free or...may I stuff there a small carpet in "U" shape, going around the vertical part of the vent?Is it necessary? Or do I just leave like it is?

It is always good to figure out what you want to do before you try to do it. ;-)

Usually, speaker enclosures are stuffed with absorptive material in order to solve an audible problem. What is your problem?

My first comment is that the box might rings like a large bell when you rap the sides of the box. You want a dull thud. Cross-braces from side-to-side, end-to-end and front-to back can address that. the braces need only be 1 1/2" x 1 1/2" .

Whatever you stuff the box with, it needs to be absorbtive at the frequency you want to damp. Carpet pretty well fails that requirement all of the time. Most box stuffing is fiberglass or cotton batting.

If you want to get an idea about the densities and thicknesses of absorptive materials to address a certain frequency, try this:

Glass fiber should not be used in vented speaker designs because there is always the danger that some fibers will be blown out of the port. Glass fibers are not as dangerous as asbestos, but getting them into your lung should better be avoided.My bass reflex speakers are damped with something like this foam - only a bit thicker:http://face.cleanandquiet.com/store/Cabinet%20Guts.jpg